1. Field of the Invention
This invention relates to a method of producing so-called SOI (Silicon On Insulator) structures in which a monocrystal semiconductor layer is formed on a monocrystal layer surface with an insulating layer interposed therebetween.
2. Description of the Background Art
In semiconductor devices, an integrated circuit having active elements which are three-dimensionally integrated to promote the degree of integration or function is referred to as a three-dimensional integrated circuit. To realize a three-dimensional integrated circuit, the technique for forming so-called SOI structures in which a monocrystal semiconductor layer is formed on an insulating layer becomes important. Among the methods of forming SOI structures is a melt recrystallization method. This melt recrystallization method is one for forming a monocrystal layer by recrystallization a polycrystal or amorphous semiconductor layer on an insulating layer by heat treatment. High output laser or electron beams are contemplated for use as energy beams, and the method using laser is chiefly used because of its improved operability. A description will now be given of a method of forming monocrystal semiconductor layers based on the melt recrystallization method using laser irradiation.
FIG. 4 is a perspective view showing the cross-sectional construction of a semiconductor device, illustrating an example of a processing step in the melt recrystallization method using laser irradiation according to the prior art. FIGS. 5A through 5C are sectional structural views showing the main step in the melt recrystallization to be described below is one using reflection-preventive film for optionally controlling the temperature distribution in a molten semiconductor layer.
First, referring to FIGS. 4 and 5A, an insulating layer 2 in the form of a silicon oxide film is formed on the surface of a silicon monocrystal substrate 1. Openings 6 are formed in predetermined regions of the insulating layer 2. The openings 6 form seed portions. Semiconductor layer, or stated concretely, polycrystal silicon layer 3 is formed on the surface of the insulating layer 2 and in the openings 6. Further, reflection-preventive films 4 of predetermined shape are formed on the surface of the polycrystal silicon layer 3. For example, silicon nitride films (SI.sub.3 N.sub.4) are used as the reflection-preventive films 4. The reflection-preventive films 4 are substantially equispaced from the seed portions 5 formed in the insulating layer 2 (see FIG. 4). FIG. 10 shows the relation between the thickness and reflectance of the silicon nitride film. As can be understood from FIG. 10, the values of film thickness corresponding to the peak and zero values of reflectance periodically appear. With this relation utilized, silicon nitride films having thickness corresponding to the two values of reflectance are selected to form a predetermined temperature distribution in the polycrystal silicon layer 3. In a conventional example, a combination of film thickness values of 0 and approximately 600.ANG. (60 nm) is selected. Therefore, in FIGS. 4 and 5A, the reflection-preventive films 4 selectively formed on the surface of the polycrystal silicon film 3 have a reflectance of approximately zero with respect to laser light 7; that is, they absorb substantially all incident light. On the other hand, the regions where the thickness of the reflection-preventive film (silicon nitride film) is zero; that is, the regions where the surface of the polycrystal silicon layer 3 is exposed, have a reflectance of about 40% with respect to the laser light 7. Thereby, the laser light incident on the entire surface of the polycrystal silicon layer 3 is absorbed well below the reflection-preventive film 4 and the region in question becomes hotter. As the laser light 7, use is made of laser light having a wavelength of 488 nm and a beam diameter of about 120-180 .mu.m. And the laser light moves at a constant speed while irradiating the surface of the substrate. The polycrystal silicon layer 3 is irradiated with the laser light 7 is heated to the molten state. The resulting temperature distribution in the polycrystal silicon layer 3 is shown in FIG. 6. FIG. 6 is a temperature distribution diagram showing the relation between positions on the surface of the polycrystal silicon layer and their internal temperatures. In the temperature distribution shown, it is seen that the temperature is lower in regions adjacent the seed portions 5.
Referring to FIG. 5B, after the laser light 7 has passed, the molten polycrystal silicon layer 3 starts to decrease in temperature, crystallizing first in the lower temperature region. As shown in the temperature distribution, the temperature is low in the vicinity of the seed portions 5; in this cooling step, recrystallization starts with the seed portions. Therefore, monocrystal regions 8a having the same crystallization as that of the monocrystal silicon substrate 1 having the seed portions 5 connected thereto spread around from the seed portions 5.
Referring to FIG. 5C, the polycrystal silicon layer having completed recrystallization changes into a homogenous monocrystal silicon layer 8. Thereafter, the reflection-preventive layers 4 are removed.
However, the monocrystal silicon layer 8 formed by the melt recrystallization method using reflection-preventive films has as undulating surface formed with coarse irregularities. FIG. 7 is a diagram showing the result of measurement of the surface irregularities of the monocrystal silicon layer 8 shown in FIG. 5C. This example of measurement shows a case where the thickness of the recrystallized semiconductor film is 550 nm. In this case, the surface irregularities are not less than about .+-.60 nm (0.06 .mu.m). The reason why the surface of the recrystallized silicon layer 8 has such undulating irregularities is that the reflection-preventive films 4 locally cover the surface of the molten polycrystal silicon layer 3. That is, those regions of the surface of the polycrystal silicon layer 3 melted by laser beam irradiation which are not covered with the reflection-preventive films 4 shrink or recess under the influence of the surface tension. This surface form is accountable for formation of undulations on the surface of the recrystallized monocrystal silicon layer 8. And such surface irregularities cause various drawbacks in forming a device on the monocrystal silicon layer 8, leading to non-uniformity of the performance of the device.
As a method of decreasing such surface irregularities, it may be contemplated to form a reflection-preventive film on the entire surface of the polycrystal silicon film 3. Referring to FIG. 10, in the above conventional example, a combination of 0 and 600.ANG. (600 nm) has been used as a combination of thicknesses of reflection-preventive films having different values of reflectance. In place of this combination, it is possible to use a combination of 1200.ANG. (120 nm) and 600.ANG. (60 nm). The melt recrystallization method using reflection-preventive films of such combination of thicknesses will now be described with reference to FIGS. 8A and 8B. FIG. 8A is a manufacturing sectional view corresponding to FIG. 5B. A reflection-preventive film 4 forms thick film regions having a thickness of 1200.ANG.(nm) at positions where they cover seed portions and thin film regions 4b having a thickness of 600.ANG. between the seed portions 5. The use of such reflection-preventive film 4 prevents the surface of the molten polycrystal silicon layer 3 from being influenced by surface tension as it is fixed to the reflection-preventive film 4. Therefore, as shown in FIG. 8B, the recrystallized monocrystal silicon layer 8 has a flat surface with limited irregularities, as compared with that shown in FIGS. 5A through 5C. FIG. 9 is a diagram showing the result of measurement of the surface roughness of the monocrystal silicon layer 8 shown in FIG. 8B. As compared with FIG. 7, it is seen that the surface irregularities have decreased to about .+-.25 nm.
However, in the case where such reflection-preventive film having thin and thick film regions is used, there arises a problem that it is impossible to form a completely monocrystalized silicon layer on the surface of the insulating layer 2 where monocrystalized regions are discontinuously formed during the recrystallization of the polycrystal silicon layer 3. This is due to the fact that an irregular surface corresponding to the grains has formed in advance on the surface of the polycrystal silicon layer 2 formed on the surface of the insulating layer 2. The irregularities on the surface of the polycrystal silicon layer 3 change the reflectance of the reflection-preventive film 4 on the surface thereof. FIG. 12 is a diagram showing the relation, found by experiments, between the thickness of a reflection-preventive film and reflected light intensity. In this figure, the solid line refers to a case where a silicon nitride film is formed on a silicon layer whose surface irregularities are about .+-.0.25 nm, and the dash-dot line refers to a case where a silicon nitride film is formed on the surface of a silicon layer whose surface irregularities are about .+-.30 nm. When the two cases are compared with each other, it is seen that in the case of the silicon layer having smaller surface irregularities, the peak value of reflectance periodically appear, whereas in the case of the silicon layer having greater surface irregularities, the peak value of reflection intensity which periodically appears with the changing thickness of the silicon nitride film gradually decreases. This is believed to be due to the fact that as the surface irregularities of the silicon layer increase, the diffused reflection of light in the interface between the reflection-preventive film the silicon layer increases, resulting in the light being confined in the silicon nitride film, whereby the reflectance decreases. The temperature distribution in the polycrystal silicon layer 3 melted by using such reflection-preventive film whose reflectance has changed is shown in FIG. 11. According to this figure, it is seen that the difference in temperature between the regions of the polycrystal silicon layer covered with the thick and thin film regions 4a and 4b, respectively, of the reflection-preventive film 4, is mild. That is, since the reflectance for laser light in the region underlying the thick film region 4a has decreased for the reason described above, the rate of absorption of laser light increases as compared with the conventional method shown in FIGS. 5A through 5C, so that the temperature in this region arises. An a temperature gradient is produced in the vicinity of the seed portions. Thus, in the case where recrystallization takes place as the temperature decreases after the laser light has passed, the solidifying temperature is reached first in the vicinity of the regions covered with the thick film region 4a and the recrystallization starts. However, since recrystallization takes place over a relatively wide area, it is not always possible to cause recrystallization to take place continuously, starting from the seed portions 5. Therefore, a discontinuous monocrystal silicon layer will be formed.